Group transmissions with a multi-gear main transmission, an upstream group in drive connection upstream from the latter and a range-change group in drive connection downstream thereof, have been known for a long time and are preferably used in utility vehicles. By designing the upstream group, which usually has two stages and can also be called the splitter group, with a small transmission ratio interval, the transmission ratio intervals between the transmission ratio steps of the main transmission are approximately halved and the total number of transmission ratio steps available is thus doubled. By virtue of a usually two-stage range-change group, the spread of the transmission, as a whole, is substantially increased and the total number of transmission ratio steps is again doubled. Hence, in combination with a three-step main transmission (with three forward gears and a reverse gear), a 12-gear group transmission (with a total of 12 forward gears and a maximum of 4 reverse gears) is obtained, and in combination with a four-stage main transmission (with four forward gears and one reverse gear) a 16-gear group transmission (with a total of 16 forward and a maximum of 4 reverse gears) is obtained.
Compared to an individual transmission with a comparable number of gears and similar gear steps and spread, the group transmission has far more compact dimensions and a lower weight. However, since many of the shift operations in a group transmission entail changing transmission ratio steps in more than one of the partial-transmissions, and therefore take place in a relatively complex manner, most of the known group transmissions are designed to be shifted by either partial or full automation.
A review of automated group transmissions by the present Applicant has been published in ATZ 9/2004 on pages 772-783. From the model series known as the AS-Ironic family of automated shift transmissions, the AS-Ironic-mid series transmissions designed for medium-weight utility vehicles and the AS-Ironic transmissions provided for heavy utility vehicles are known, in each case designed as group transmissions with a multi-step main transmission, i.e., one comprising three or four forward gears, a two-stage splitter group upstream from the main transmission, and a two-stage range-change group downstream from the main transmission.
In each case, the main transmission is of a countershaft design, being provided with unsynchronized claw-type clutches and, in the case of the AS-Tronic-mid series, comprises a single countershaft. For reasons to do with weight and structural space optimization, the transmissions of the AS-tronic series have two countershafts. In both series, the main transmission is optionally available in a direct-gear version (iHG—min=1) or in a fast-gear version (iHG—min<1).
In each case, the splitter group is designed as an upstream transmission with two shiftable input constants for the main transmission. In each case, the range-change group is designed as a two-stage planetary transmission which can be shifted between a direct connection mode (iBG=1) and an alternative, high transmission ratio (iBG>>1).
Other group transmission designs, in each case with a main transmission, an upstream splitter group and a downstream range-change group, are known from DE 101 43 994 A1, for example.
In the group transmissions of the AS-Ironic and AS-Tronic-mid series, until now the shifting clutches of the splitter group and of the range-change group, in each case combined in a common shifting packet, are all of synchronized design, while in contrast the main transmission is designed for claw-type shifting, i.e., to be shifted without synchronization. However, since synchronized shifting clutches are expensive, because of their complex structure, take up a relatively large amount of structural space, and limit the life of the transmission as a whole because of wear, it is intended in future designs of such group transmissions to design the range-change group, in addition to the main transmission, with claw-shifting means.
By way of example, FIGS. 3a and 3b show schematically the structure of the group transmissions of the AS-Ironic series known per se. The main transmission HG is made as a direct-gear transmission of countershaft design and has a main shaft W2 and two countershafts W3a, W3b. In the version shown in FIG. 3a, the main transmission HG is a four-step transmission with four transmission ratio steps G1 to G4 for forward driving and a transmission ratio step R for reversing. In the version shown in FIG. 3b, the main transmission HG is formed as a three-stage transmission with three transmission ratio steps G1 to G3 for forward driving and a transmission ratio step R for reversing.
The loose wheels of the transmission ratio steps G1, G2, G3, R and G1, G2, R, respectively, are in each case mounted to rotate on the main shaft W2 and can be engaged by means of associated claw clutches. The associated fixed wheels are arranged rotationally fixed on the countershafts W3a or W3b. The highest transmission ratio step G4 or G3, respectively, in each case made as a direct gear, can be engaged by means of a direct-shift clutch. So far as possible, in each case two shifting clutches at a time are combined in a common shifting packet S1/2, S3/4 or S1/R, S2/3. In the version shown in FIG. 3a, the shifting packet SR comprises only the shifting clutch of the transmission ratio step R for reversing.
The upstream group VG is of a two-stage design and is also made with countershafts, such that the two transmission ratio steps K1 and K2 form two shiftable input constants for the main transmission HG. By virtue of a small ratio difference between the two transmission ratio steps K1, K2, the said upstream group VG constitutes a splitter group. The loose wheel of the first transmission ratio step K1 is mounted to rotate on the input shaft W1, which is connected to a drive motor, in the form of an internal combustion engine, by a controllable separator clutch located outside the area covered by the representations shown in FIGS. 3a and 3b. The loose wheel of the second transmission ratio step K2 is mounted to rotate on the main shaft W2. The fixed wheels of the two transmission ratio steps K1, K2 are respectively arranged rotationally fixed to the countershafts W3a, W3b on the input side. The shifting clutches of the splitter group VG, which are of a synchronized design, are combined in a common shifting packet SV.
The range-change group BG, in drive connection on the downstream side, is also of a two-stage design, but is a planetary structure with a single planetary gearset. The sun gear PS is rotationally fixedly connected to the main shaft W2 on the output side. The planetary gear carrier PT is rotationally fixedly connected to the output shaft W4 of the group transmission. The annular gear PH is connected to a shifting packet SB comprising two shifting clutches, by means of which the range-change group BG can be shifted alternatively to a slow-drive stage L, by connecting the annular gear PH to a fixed part of the housing, or to a fast-drive stage S, by connecting the annular gear PH to the planetary gear carrier PT. In contrast to the previous design mode of AS-Ironic transmissions, the shifting clutches of the shifting packet SB are, in this case, of unsynchronized design.
In a group transmission of this type, owing to the unsynchronized design of the range-change group BG, a special shifting sequence is needed, which is distinctly different from that of a largely identical group transmission, but one provided with a synchronized range-change group BG. A suitable method for controlling shifts, in a group transmission with an unsynchronized range-change group BG, is known from DE 101 52 857 A1, for example. In this method, it is essentially provided that for a range shift, the splitter group VG and the range-change group BG are each first shifted to their neutral position in order to interrupt the force flow, the main transmission HG is then braked by a transmission brake, and the adjustment of the speed of the drive motor to the synchronous speed of the target gear begins. After the shifting of the main transmission HG, the splitter group VG is shifted by synchronization to its target transmission ratio. When the drive motor has reached the synchronous speed, the target transmission ratio of the range-change group BG is engaged.
Since in a range shift, the main transmission HG is shifted in the opposite direction to the range-change group BG, i.e., for an upshift of the range-change group BG, from the slow-driving stage L to the fast-driving stage S, the main transmission HG is shifted from a high transmission ratio, such as G4 or G3, to a lower ratio, such as G1 or G2, with the arrangement of transmission ratio steps in ascending or descending sequence as has been usual until now, a change is always necessary between two shift gates and the shifting packets associated with these. This relationship is summarized in the tables of FIGS. 4a and 4b, for the group transmissions shown in FIGS. 3a and 3b for various gear intervals. Since an automated gate change, in each case, entails controlling and stopping the associated adjusting drive, sensing that the first shifting packet is in the neutral position, switching between the two gates or shifting packets and controlling and sensing the adjusting drive in the target shift position(s) of the second shifting packet, a gate change has a disadvantageous delaying effect on the shift sequence as a whole.